Membership and submit

As it’s demo site, membership levels are disabled on it. For membership I can recommend plugins S2Member (free version is available) or Ultimate Membership PRO. But you can use any of plugins for this.

Theme also has some helpers for membership. First of all, you can use it’s login/register popup and some shortcodes (check below) + it has full support for Buddypress if you need super powerful user functions including private messages, profiles, friendship, following and activity. Check demo of Recash to see BuddyPress customizations of theme. Also theme supports MyCred and badges, ranks, levels, etc. Also theme has powerful user dropdown and user menus, support for social login. Check some docs